A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol, e.g., a methanol (feed) composition, with carbon monoxide. The catalyst may contain rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises acetic acid, methyl acetate, water, methyl iodide and the catalyst. The methanol and the carbon monoxide come into contact in the reaction medium and react with one another to form crude acetic acid. Conventional commercial processes for the carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entireties of which are incorporated herein by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entirety of which is incorporated herein by reference.
Typically, crude acetic acid is separated using a separation train having inter alia a carbonylation reactor, a flash vessel, a light ends column, and a decanter. U.S. Pat. No. 7,855,306, the entirety of which is incorporated herein by reference, discloses a process, e.g., a process comprising a separation zone, for reduction and/or removal of PRCs from a carbonylation product by (a) separating the carbonylation product to provide a vapor overhead stream; (b) distilling the vapor overhead stream to yield a low boiling overhead vapor stream; (c) condensing and separating the low boiling overhead vapor stream to form a condensed light liquid phase; (d) distilling the condensed light liquid phase in a single distillation column to form a second vapor phase stream enriched with PRC's; and (e) condensing and extracting the second vapor phase stream with water to obtain an aqueous acetaldehyde stream comprising PRCs.
U.S. Pat. No. 8,940,932 discloses a process for stably producing high-purity acetic acid while efficiently removing acetaldehyde. The process for producing acetic acid comprises a reaction step for allowing methanol to react with carbon monoxide in the presence of a metal catalyst, a halide salt, and methyl iodide; a step for continuously feeding a flash vessel with the reaction mixture and separating a lower boiling point component containing acetic acid and methyl iodide and a higher boiling point component containing the metal catalyst and the halide salt; a step for feeding a distillation column with the lower boiling point component, and separating a lower boiling point component containing methyl iodide and acetaldehyde and a stream containing acetic acid to collect acetic acid; a condensation step for condensing and temporarily holding the lower boiling point component in a decanter and discharging the lower boiling point component from the decanter; and a step for separating the lower boiling point component discharged from the decanter into acetaldehyde and a liquid residue and recycling the liquid residue to the reaction system. In the condensation step, the amount of the lower boiling point component to be held is controlled based on a fluctuating flow rate of the lower boiling point component to be fed to the decanter.
While the above-described processes have provided some processes for separating crude acetic acid, specifically for maintaining the liquid level in a decanter/condenser, these processes establish control by measuring the light ends overhead stream and then varying the recycle streams to the reaction zone, which may create problems with respect to the overall water balance in the reaction system and, in some cases may provide for insufficient control. These methods may also require the use of additional process components or the modification of process components, e.g., decanters with buffering capability. Thus, the need exists for improved processes for maintaining consistent light ends and decanter operations, which provide for: 1) a more consistent purified product composition e.g., a more consistent light ends column as a sidedraw composition, and 2) more precise liquid level control without disturbing the water balance in the reaction system or requiring additional process components or process modifications.